This invention relates to a patch for repairing both sides of a panel, one side of which is inaccessible, and more particularly to a repair patch for such panels made of composite material.
There are a great many structures made with panels which are accessible only from one side. These include vehicles of all varieties, such as ground and water vehicles, aircraft and even space vehicles, and also include fixed structures such as buildings and storage tanks. When damage occurs to these panels, the damage must be repaired to restore the structural strength of the panel, restore its smoothness and appearance, and in the case of aerodynamic surfaces, reconfigure the surface to its original curvature or configuration, as close as possible to the original mold line. Repairing the accessible or near-side of the panel is seldom difficult but the panel cannot be restored to full strength if the inaccessible or blind-side of the panel is not repaired. Such blind-side repairs are difficult because of the limited access for positioning the patch material and applying pressure while it cures in place. A panel patched only on its near-side is weak on the blind-side because it is unable to transfer loads across the damaged section. When such a panel is subjected to a bending moment, it is subject to failure at loads much lower than the design capacity of the panel.
The problem is particularly troublesome with modern composite materials because of the higher load requirements for these materials and because of the necessity for producing flush repairs without the use of fasteners. Fasteners were permitted in prior art near-side patches but are undesirable for modern composite material patches. To accommodate the fasteners, additional holes must be drilled in the material, which creates additional weakness in the panel. Moreover, it is difficult to produce a patch that is flush with the original mold line of the panel and conforms exactly with its curvature when using a patch secured with fasteners. Finally, such fasteners exert a constant compression force on the repaired composite panel or honeycomb panel to hold the patch in place which could distort the cross section of the panel in the patch location and thereby reduce its load carrying capacity.
The installation of prior art patches in the vicinity of underlying spars and ribs within the member on the blind-side of the panel requires specially designed patches or connecting structure to the struts or ribs which could interfere with underlying mechanisms. On panels which form an aerodynamic surface, especially on high performance aircraft, the non-conformance to the original contour and loss of smoothness could produce a flutter of the control surface which could adversely affect aircraft performance and cause premature fatigue delaminating of the panel structure.
The few prior art patches for composite materials which provide for patch material on the blind-side require a series of steps, each requiring a separate cure time for the bonding agent in each of the steps. This prolongs the repair process and increases the number of steps involved in making the repair, thereby increasing the cost and increasing the down time of the equipment being repaired. In addition, it is an exacting process requiring special skills and care to produce an acceptable patch. Moreover, the prior art blind-side patches require complicated expensive and cumbersome equipment requiring special training of operators and subject to failure in operation which would interrupt the capability of the facility to make those repairs.
Some other unsolved problems with prior art patches have been the inability to effect a permanent hermetic seal against leakage of pressurized fuel, air, water and other fluids and gases through the patch. Also, prior art patches are often thermally or chemically incompatible with the parent structure so that differential rates of thermal expansion cause loss of hermetic integrity of the patch or cause the panel to become distorted. Chemical incompatibility could cause loss of chemical resistance to certain chemicals such as hydraulic fluid which could cause the patch to swell and weaken. In addition, dissimilar metals and carbon/metal combinations can cause a harmful galvanic reaction that would weaken the patch.